The present invention relates particularly to a method and apparatus for measuring (quantifying) the depths of cracks in workpieces through the use of eddy currents measured in workpieces.
The use of eddy current testing technology to detect and locate anomalies in materials is well established. Eddy current testing is the process of inducing small electrical currents by a device into an electrically conductive workpiece and observing the resultant reaction between the magnetic fields involved. Cracks in conductive materials affect the flow of these eddy currents that can be detected and related to the depth of a crack.
By way of further background, eddy currents are created in a workpiece when alternating current flows in a coil in close proximity to a conducting surface of the workpiece. The magnetic field of the coil will induce circulating eddy currents in that surface. The magnitude and phase of the eddy currents will affect the loading on the coil and thus its impedance. When a flaw is detected in the surface of the material, the eddy current flow will be interrupted or reduced, thus decreasing the loading on the coil and increasing its effective impedance.
Though conventional eddy current technology is capable of detecting cracks and/or flaws in conductive material, it has not been used to measure the depth of a crack in a non-destructive manner. The traditional method used to measure crack depth is to mechanically open the crack or cross-sectioning through the crack. This approach is destructive because of the damage done to the workpiece in determining the crack depth. Therefore, it is ineffective, time consuming and costly. A need exists for a technique whereby the depth of a crack can be measured accurately without destroying the workpiece.
The present invention overcomes the above-described drawbacks through an apparatus and method that non-destructively measures surface crack depths on components with precision and accuracy using an eddy current testing process. This non-destructive process is beneficial whereby eliminating the need to destructively cross-section through the crack. In addition, machining an amount of material as determined by this invention might repair components exhibiting crack indications. This method utilizes established eddy current instruments and a known (i.e., previously empirically determined) relationship between eddy current response and crack depth. The crack is initially detected and located by proven non-destructive methods such as but not limited to magnetic particle inspection, fluorescent penetrant inspection, eddy current testing, and/or visual or microscopic examination. The eddy current response, which by way of example may be a real-time display of the probe impedance in the form of, again by way of example, a refreshing dot on a display screen, indicates a change in impedance, visually manifested by way of example as a vertical rise and drop, or trace, of the aforementioned dot, as a probe of a measuring instrument passes over the surface to be evaluated. When a crack is not present and the material is absent of other anomalies affecting eddy currents, the impedance is constant and there is effectively no change in the signal, or no eddy current response, manifested by way of example as a stationary or tightly-tracing dot in the aforementioned example of response display. However, if there is a crack (or other anomaly that affects eddy currents) then the impedance changes. Crack responses result in mostly vertical signal changes when the measuring device is so altered. The magnitude of a signal change due to a crack, referred to henceforth as xe2x80x9cresponsexe2x80x9d, is proportional to the depth of the crack.
The relationship between the eddy current response and crack depth is determined empirically by generating a table of data points with each point defined by two parameters: 1) maximum eddy current signal response measured from the crack, and 2) actual depth of said crack from 1) measured with the proven method of mechanical opening. Each data point represents a unique crack and the accumulation of many data points defines a curve representing the relationship between eddy current response and crack depth. The curve is relevant to a specific material. The curve is then applied as a tool to predict the crack depth in a component of the same material from the measured eddy current response in a non-destructive manner. The term unique, as in reference to a crack, as used herein may by way of example refer to 1) cracks in, the same workpiece separated by a distance or, 2) to cracks in different workpieces of the same material or, 3) to several cracks originating from (that is, at one time having been part of) a single original crack where the original crack having surface material removed (thereby effectively decreasing its depth) effectively becomes another crack, shallower and unique to the original (deeper) crack, from the perspective of eddy current response.